Polyamine biosynthesis will be investigated after fertilization and during preimplantation stages of mouse embryo development. Measurements will be made of ornithine decarboxylase (ODC), and S-adenosyl-L-Methionine decarboxylase activities in addition to intraembryonic levels of putrescine, spermidine and spermine. The effects of these compounds on development will be noted. Changes in ODC activity and polyamine concentrations will be correlated with changes in the syntheses of macromolecules and with changes in cyclic nucleotide metabolism.